The bike fitting was once a mysterious process. You'd get your limbs measured, saddle up, and ride off. But there are so many variables in a bike's fit, and some have nothing to with the length of your limbs.

Fresh ideas on how to measure a rider's needs has given rise to a new breed of bike fitter. One of the most sophisticated labs for this kind of cycling science is at Acme Bicycle Co., a Brooklyn, New York shop run by Jonathan Blyer. Gizmodo visited Acme for a data-driven fitting, and a closer look at how the mix of robots, computers, and motion-capture video come together to find you the perfect bike.

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There are a lot of reasons for a regular rider to get a fitting. Maybe you're looking for a new bike and you want to get the perfect size for your body. Maybe you want to optimize your current ride for racing (or commuting, or whatever). Maybe you've decided that you're going to get your first custom bike, and you want it precisely tuned to fit your strength and riding style. Here's a look at how a fitter like Blyer will figure out your needs.

1. Goals and history. Blyer interviews you about to figure out how you like to ride. A bike designed for long rides in the country is going to look very different than one designed for triathlons.

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2. Flexibility and strength. After tests in several key areas, Blyer takes some measurements. He looks for asymmetries in your body, and asks about lingering injuries. Some things can be compensated for in your bike's setup. For example, if you have lower back problems, he's probably going to have your handlebars raised a bit higher to alleviate some of that pressure.

3. How you ride. Blyer puts you on a stationary bike and watches you ride. But oh, this isn't any normal stationary bike. This is the Guru Dynamic Fit Unit (DFU), a fully mechanized and computer-controlled bike analog. It may not look like a bike, but it feels just like one when you're riding it. If you come in with your own bike, Blyer will have measured it and set up the DFU to match it exactly (right down to your seat, handlebars, crank-size, and pedals). That's your starting point. Then, as you ride, he can tweak the positioning of the handlebars and seat up/down and forward/back. This process used to take forever on a manually adjusted fit bike, because every time you wanted to adjust something you had to get off of it. With the DFU you don't even have to stop pedaling, and you can adjust in increments down to the millimeter.

As he makes adjustments, he watches for changes in your body mechanics. Hips moving too much? Maybe the seat is just a hair too high. Too much curve in your back? Maybe bring the handlebars forward. He tweaks and tweaks until he has what looks like an optimized position for what you want. But looks can be deceiving, so he goes in deeper for some serious metrics.

4. The way you move.The Retül system is some sci-fi goodness. The system's head looks like a gigantic Xbox Kinect. It points at the rider, who wears eight LEDs placed at very precise points on his/her body. (That's why I had to wear the Spandex shorts and tanktop, and I'm sorry you all had to see that.) This system gives you the raw data that your eyes can't see, even if you were looking at slow motion video. It calculates all of the relative angles in your body down to one millimeter, and it works three dimensionally. If your knee isn't extending enough, you can tweak the seat millimeter by millimeter and fix it. If your knee is following a pronounced elliptical pattern (e.g. it pops out to the side on your down stroke then comes in on your upstroke, or vise versa) the Retül sees that, and you can work on correcting it from there. It really helps you see how your right and left sides are (or aren't) balanced. The fit gets dialed in even more precisely from there.

5. The specs on the perfect bike.

Once the fit is optimized, Blyer uses another tool for the Retül system called the Zin. It's essentially a gauge with four LEDs you use to trace the bike's outline at key points. Blyer drags the Zin over the seat, the handlebars, around the crank, etc. Those numbers go into a computer, and it creates a database of measurements. Those measurements would then be applied to your current bike, or they could be used to create your perfect custom bike. For example, here's what my custom Guru Photon frame would look like, which I will be buying just as soon as I stub my toe on a $5,000 gold nugget.

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6. Some key adjustments. Even without placing an order for a custom bike, the fitting will help you optimize your current ride. We found that I was a bit bunched up on my bike. Blyer raised the seat, moved it forward, and put on a longer handlebar stem on there so I could extend out further. I instantly felt that I had way more leverage and power, and it was easier to maintain a comfortable position (I had a tendency to hunch over before). My thighs weren't hitting me in the gut, and I didn't have too much weight on my hands. It was like riding a new bike. Really remarkable.

7. Worth it? The whole process can take three to four hours, and it'll run you a few hundred bucks. Obviously, this isn't something you'd do to make sure your Huffy Spiderman bike fits just right, but if you're about to drop a serious chunk of change on something new (stock or custom), a process like this is definitely worth it. By using cold, hard numbers instead of simply gut feelings, the data can be shared with any bike builder in the world to get the right product for your needs. This level of precision changes the way bikes are built and fitted, and the more widespread this kind of technology becomes, the faster, smoother, and healthier we'll all be riding.

Big thanks to Jonathan Blyer for taking the time to help us with this. Only a small handful of guys use the Guru DFW and the Retül system together, so if you're in the Northeast and you want a serious bike fitting, he's your man. Check him out at Acme Bicycle Co. Thanks also to Phil Pinsky from Guru for coming down from the frosty north.